(1) Field of the Invention
The present invention relates to a semiconductor integrated circuit device and a vehicle-mounted radar system using the same, and more particularly relates to a monolithic microwave integrated circuit (MMIC) device in which an active circuit and a passive circuit are monolithically integrated.
(2) Description of Related Art
High-frequency parts used for mobile communication systems typified by cellular phones or vehicle-mounted radar systems have been demanded to have improved performance and a reduced size.
Conventionally, field-effect transistors (FETs) or heterostructure field-effect transistors (HFETs) both made of gallium arsenide (GaAs) have been used as electronic devices having high-output characteristics, low-noise characteristics and high-gain characteristics.
In recent years, attention has been paid to devices using an unconventional material of gallium nitride (GaN), as devices that can be expected to operate at a higher output, a higher frequency and a higher temperature than the devices using GaAs. GaN has a large band gap of 3.39 eV, and the dielectric breakdown voltage of GaN is about one order larger than that of GaAs. This increases the saturation drift velocity for electrons. Since M. Asif Khan et al. realized electronic devices using AlGaN/GaN-based compound semiconductor, the development of GaN-based devices has been advanced (see, for example, M. Asif Khan, “High electron mobility transistor based on a GaN—AlxGa1-xN heterojunction (Appl. Phys. Lett., 63(9), 30 (1993), pp. 1214-1215).
In view of the previously-mentioned characteristics, GaN-based devices are promising as basic devices forming RF front-end sections of radio communication systems or radar systems. As previously mentioned, GaN-based devices using GaN having a large band gap exhibit high-breakdown-voltage characteristics. Therefore, it becomes unnecessary for each of GaN-based low-noise devices forming a reception circuit to have an input protection circuit that has been required to cope with an external surge, resulting in the reduced insertion loss of the protection circuit. As a result, the noise figure can be reduced. Furthermore, input power can be set high such that an interference wave produced at the output side with increase in an input signal becomes ignorable and the voltage of the input power does not exceed the breakdown voltage of the GaN-based low-noise device. Therefore, GaN-based low-noise devices also exhibit low-distortion characteristics.
GaN-based high-output devices forming transmitting circuits have high saturated output power and a high linearity of input-output characteristics between input power and output power. As seen from the above characteristics, mixers used as frequency converters or switches used as signal switches also exhibit low-loss and low-distortion characteristics.
As seen from the above, the GaN-based devices have a high sensitivity to weak input signals and low distortion characteristics even with high input power and are useful as devices that can deal with signals having a wide dynamic range.
By the way, a Group III-V nitride semiconductor layer made of GaN is epitaxially grown on a substrate made of sapphire (monocrystalline Al2O3) or silicon carbide (SiC) by vapor-phase epitaxy (VPE) such as metal organic vapor phase epitaxy (MOVPE) or molecular beam epitaxy (MBE).
A known GaN-based device will be described hereinafter.
It is typical that in the GaN-based device, a passive circuit, such as an inductor or a capacitor, is formed on an isolation region obtained by insulating part of a conductive layer located around an active element, such as a transistor, or formed with a dielectric film of silicon nitride (SiN) or the like interposed between the passive circuit and the isolation region (see, for example, the above paper).
Furthermore, as shown in FIG. 7, a monolithic microwave integrated circuit of a first known example in which an active element, such as a GaN-based transistor, and a passive circuit, such as a matching circuit or a bias circuit including an inductor, a capacitor, a resistor element, or a distributed-constant circuit, are integrated on a single substrate takes on a structure in which an isolation region 110 forming an insulated compound layer formed by ion implantation or the like is formed between the passive circuit and, for example, an insulative sapphire substrate 101.
To be specific, in the known GaN-based device, a GaN-based semiconductor layer 105 including a buffer layer 102 made of undoped AlN, a channel layer 103 made of undoped GaN and a carrier supplying layer 104 made of n-type AlGaN is formed on the principal surface of a sapphire substrate 101 by MOVPE. A gate electrode 106 is partly formed on the GaN-based semiconductor layer 105, and a source electrode 107 and a drain electrode 108 are formed on parts of the GaN-based semiconductor layer 105 located to both sides of the gate electrode 106, respectively, thereby forming a HFET 109.
An isolation region 110 is formed in the side portion of the GaN-based semiconductor layer 105 located to the drain electrode 108 side by implanting, for example, boron ions thereinto. An insulating film 111, for example, made of silicon nitride is formed on the GaN-based semiconductor layer 105 and the isolation region 110 to cover the gate electrode 106, the source electrode 107 and the drain electrode 108.
Furthermore, a conductive film 112 is entirely formed on the (back) surface of the sapphire substrate 101 opposite to the GaN-based semiconductor layer 105.
An interconnect 115 connected at one end to the source electrode 107 to pass through a first through hole 113 and at the other end to the conductive film 112 to pass through a second through hole 114 and a microstrip line 117 connected at one end to the drain electrode 108 to pass through a third through hole 116 are formed on part of the insulating film 111. In this case, the microstrip line 117 uses the conductive film 112 formed on the back surface of the sapphire substrate 101 as a ground line. The first and third through holes 113 and 116 pass through the insulating film 111, and the second through hole 114 passes through the insulating film 111, the GaN-based semiconductor layer 105 and the sapphire substrate 101.
FIG. 8 shows a GaAs-based MMIC of a second known example including a transistor section 5a, a capacitor section 5b and an antenna pattern all formed on a sapphire substrate 1, and an interconnect 4 through which they are connected to one another (see, for example, Japanese Unexamined Patent Publication No. 5-243843). At least one of the antenna pattern and the interconnect 4 is formed of an oxide high-temperature superconductor. Each of the sides of the transistor section 5a and the capacitor section 5b is partly covered with silicon oxide 6, and normal interconnects that are not made of a superconductor are formed to extend from part of the principal surface of the sapphire substrate 1 located in the transistor and capacitor sections 5a and 5b to part of the top surface of a conductive n-type GaAs layer 3b. 
However, the GaN-based MMIC of the first known example shown in FIG. 7 using, for example, a microstrip line 117 for a passive circuit has the following problems. It is difficult that when an isolation region 110 is formed by insulating part of a GaN-based semiconductor layer 105 by ion implantation, its resistance is uniformly increased also for its region located in the vicinity of the upper part of a sapphire substrate 101. When a passive circuit is formed immediately above the isolation region 110 whose resistance is not uniformly increased, this makes the high-frequency loss larger as compared with the case where a passive circuit is formed on a uniformly insulated substrate. Furthermore, when the insulation performance of a substrate that determines high-frequency characteristics is not uniform, this reduces the permittivity of the substrate as compared with the case where a substrate is uniformly insulated. The reduction in the permittivity of the substrate increases the physical length of the microstrip line 117 required to achieve a desired electric length. As a result, the area of an MMIC chip is increased, leading to problems in downscaling the chip. The increase in the area of the chip becomes one of factors responsible for the increased cost.
Since in the GaAs-based MMIC of the second known example shown in FIG. 8 an interconnect is formed directly on a conductive layer (a silicon layer 3a or an n-type GaAs layer 3b), a dielectric loss due to the conductive layer and a high-frequency loss due to an impedance mismatch are caused in the interconnect serving as a high-frequency transmission line.
Moreover, since the interconnect is formed directly on the top surface of the conductive layer and some of the side surfaces thereof, the interconnect is likely to be partly removed at the corner (bend) of the conductive layer in the formation of the interconnect.